Artificial turf was popular in the early 2000s, and schools with money covered their grounds with the artificial greenery. The Ministry of Education offered financial help to schools to make the switch, and more than 1,000 schools across the country joined in the change.

Artificial grass was welcomed because it dried fast after rain, and protected students better from injuries.

However, health risks have become an issue, according to the Ministry of Education.

"Concerns have been raised over toxic chemicals used in the artificial turf and their impact on children's health," a ministry official surnamed Lee said. "It was also expensive to maintain the artificial grass, the lifecycle of which was between seven and eight years, and it required endless maintenance."

The ministry stopped its financial support in 2012.

Last year, the ministry conducted a national investigation and found 174 out of 1,037 schoolyards had at least one of 11 toxic chemicals in its artificial grass, which was higher than permitted amounts. The toxic substances include lead, cadmium, mercury, hexavalent chrome and benzopyrene.

At one school in Pohang, the amount of benzopyrene was 470 times higher than the permissible level, which is 1 milligram per kilogram.

Following the investigation, the Busan Metropolitan Office of Education ended its support for artificial turf. Last year, it paid all the expenses to have 11 schools in the city remove their artificial grass, after the ministry's study showed that the artificial turf at the schools was found to contain the toxic substances. It will continue to pay for schools up to 200 million won to remove the turf.

"It will take time because we work with a limited amount of resources," said an official from the Busan office, "but we'll get it done."

Last year, the Seoul Metropolitan Office of Education also started recommending schools opt for dirt playgrounds.